1. Field of the Invention
The present invention relates to a light guide plate, more particularly to a light guide plate with an excellent appearance and redeced irregularity in luminance, in particular a thin-walled, large screen size light guide plate.
2. Description of the Related Art
A light guide plate is one optical member used in a backlight unit mounted in various display devices. For example, an edge light type planar light source device is generally comprised of a light guide plate for guiding light from a light source entering from a side end face in a direction parallel to the plate face and causing it to be emitted in a direction substantially perpendicular to the plate face, a reflector arranged so as to surround the light source for efficiently guiding the light of the light source not directly entering the light source-side end face of the light guide plate, a light diffusion sheet or plate arranged at the light emitting face side of the light guide plate for causing diffusion of light emitted from the emission face, and a reflection sheet or plate arranged at the light reflection face side of the light guide plate for returning the light leaked from the light guide plate to the light guide plate once again.
Note that the back face of the light guide plate (light reflection face) is formed with a pattern of various shapes such as dots, cone cuts, and V-grooves for raising or uniformly diffusing the luminance of the light introduced into the light guide plate.
The light guide plate has to have redeced irregularity in luminance since the entire emission face serves as the direct light source of various types of display devices. Further, it is believed desirable that the color temperature be high. Therefore, in the past, use has been made of a light guide plate made by injection molding a colorless transparent thermoplastic resin such as polymethyl methacrylate (PMMA) or polycarbonate (PC). Further, recent light guide plates have tended to be made thinner from the viewpoint of increasing the screen size or economizing the space.
The PMMA normally used for molding a light guide plate, however, has a high melt viscosity at the time of injection molding and inferior fluidity and is difficult to mold into a thin-walled, large screen size of over 10 inches, further over 14 inches. Even if able to be molded, since it is thin, there was the problem that hygroscopic deformation ended up occurring. On the other hand, if the resin temperature is raised to improve the fluidity, the resin is liable to foam in the cylinder and a shaped article with a good appearance is difficult to obtain such as due to the occurrence of voids. Further, since PC has a high heat deformation temperature, the molding temperature has to be raised in order to obtain a fluidity sufficient for molding thin articles. As a result, due to the effects of moisture absorption, the resin is liable to hydrolyze and foam in the cylinder. In the same way as the case of PMMA, voids occurred in the shaped article (light guide plate) and it was difficult to obtain a shaped article with a good appearance. Therefore, a molding material has been sought which enables molding of a thin-walled light guide plate of a large screen size of over 10 inches or over 14 inches with a good appearance by injection molding.
Further, when using PMMA or PC as the molding material and forming V-grooves etc. in the back face of the light guide plate, there has been the problem that it was not possible to precisely transfer the pattern of the fine shapes to the light reflection face side of the thinnest part, that is, the narrow end portion, in a wedge-shaped light guide plate where the thickness of the light guide plate becomes gradually thinner the further from the light source. This becomes a cause of a reduction of the homogenity of the light emitted due to luminance unevenness.
An object of the present invention is to provide a light guide plate of a good appearance and redeced irregularity in luminance, in particular a thin-walled, large screen size light guide plate, and a process for production of the same.
The present inventors took note of the melt fluidity of a thermoplastic resin and engaged in intensive studies with the aim of improving it. As a result, they discovered that by melt molding a thermoplastic resin having a specific melt flow rate (hereinafter also referred to as an xe2x80x9cMFRxe2x80x9d), it is possible to obtain a light guide plate with a good appearance and redeced irregularity in luminance, in particular a thin-walled, large screen size light guide plate, and thereby completed the present invention.
A light guide plate according to the present invention is characterized by being obtained by melt molding a thermoplastic resin having a melt flow rate of at least 50 [g/10 min.] under a load of 2.16 kgf at 280xc2x0 C.
A process of production of a light guide plate according to the present invention is characterized by melt molding a thermoplastic resin having a melt flow rate of at least 50 [g/10 min.] under a load of 2.16 kgf at 280xc2x0 C.
Effects
According to the present invention, there is provided a light guide plate with a good appearance and redeced irregularity in luminance, in particular a thin-walled, large screen size (for example, at least 10 inches size, preferably at least 14 inches size) light guide plate, and a process for production of the same.
In particular, a thermoplastic resin having an MFR of at least 50 [g/10 min.] has a low viscosity. Therefore, according to the present invention using such a resin, the melt fluidity of the resin at the time of melt molding can be improved and a shaped article (light guide plate) having a good appearance can be obtained. In particular, if a resin with a low viscosity is used when molding a thin-walled, large screen light guide plate, fluidization and plastization become possible even at a low temperature and cooling and solidification become easy. Further, since the melt fluidity is good, it is possible to precisely transfer even a pattern of fine shapes such as V-grooves to the reflection face side of the light guide plate. Further, the cycle time at the time of molding is comparatively short, the productivity of the light guide plate rises, the residence time in the molten state becomes shorter, and the rate of occurrence of voids, burn marks, and discoloration falls. Therefore, it becomes easy to obtain a light guide plate with a good appearance and redeced irregularity in luminance even when producing a thin-walled, large screen size (for example, at least 10 inches size or at least 14 inches size) light guide plate.